This invention relates generally to signal processing systems and more particularly to beamforming controls for phased array antenna systems.
Phased array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front, or the cumulative wave front of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna activating signals determines the direction in which the cumulative beam from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similary allows determination of the direction from which a return beam arrives.
In a phased array antenna system used as a radar, for example, it is desirable to control the electromagnetic energy pattern in two dimensions (i.e., along two axes of the antenna array that are orthogonal to one another), typically identified as elevation and azimuth. In most radar systems it is further desirable to rapidly scan the elevation as the radar system simultaneously scans the azimuth. Thus radar system performance can be enhanced with a beam control system that allows for a very rapid scanning along one axis (e.g., the vertical axis or elevation) while still having a rapid and reliable scanning along the other axis (e.g., the horizontal axis or azimuth).
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antennas (or subarrays of antennas), has conventionally been accomplished by electronically shifting the phase of actuating signals or by introducing a time delay in actuating signals for selected antenna elements to sequentially excite the antenna elements to generate the desired direction of beam transmission from the antenna. Opto-electronic processing of beamforming signals for phased array antennas offers numerous advantages over conventional electronic-only processing by reason of superior performance of optical control systems, including increased bandwidth, compactness, and signal stability. Examples of such opto-electronic systems are disclosed in U.S. Pat. No. 5,117,239 of N. Riza issued May 26, 1992, and in the copending applications referenced above, all of which are assigned to the assignee of the present invention and are incorporated herein by reference.
Ideally, a phased array antenna control system should have the ability to rapidly scan in elevation for a given azimuth scan rate, and should be light, compact, relatively immune to undesirable electromagnetic radiation, and straightforward to fabricate, operate, and maintain. Such a system also desirably has a wide antenna tunable bandwidth, and inertialess, motion-free high resolution beam scanning ability with application-dependent slow-to-fast scanning speeds.
It is additionally advantageous to have an analog beamforming control system that allows a large number of possible phase shift combinations. Such an analog system is in contrast to digital phase control from electronic phase shifters, which provide a fixed number of possible phase actuation signals. This limited number of possible actuation signals in turn limits the phase resolution achievable with the microwave devices, thus limiting the angular resolution of the scanned antenna beam. Further, in conventional electronically controlled phased arrays, the digital microwave phase shifters are also typically used for correcting phase errors that result due to the other microwave devices in the system. Because of the digital nature of the phase shifters, the phase errors can only be partially cancelled.
Optical control systems can be advantageously used to generate control signals for phased array antennas. For example, an optical control system can use heterodyne detection between respective phase-shifted light beams in an optical signal pair to generate a scanning interference phase pattern. Such a system, using a liquid crystal pixel array for generating phase delays in one light beam of an optical signal pair is disclosed in commonly assigned, application U.S. Pat. No. 5,191,339. Another method of generating a scanning pattern based on interference between light beam pairs is described in the article authored by N. Riza entitled "An Acoustoopic Phased Array Antenna Beamformer with Independent Phase and Carrier Control Using Single Sideband Signals", appearing in IEEE Photonics Technology Letters, Vol. 4, No. 2, February 1992.
It is accordingly an object of this invention to provide an optical signal processing system for controlling scanning arrays that can simultaneously generate two-dimensional analog phase-based modulo 2.pi. phased array antenna beam control signals.
It is a further object of this invention to provide a phase-based antenna controller that is relatively compact, lightweight and has an inertialess two-dimensional beam scanning structure.
Another object of this invention is to provide a two-dimensional phase-based antenna controller that has a wide (i.e., in the GHz range) tunable antenna bandwidth with stable phase-control and an independent, analog, phase-error calibration capability for all the elements in the array.